A laser beam can be readily expanded into a line by passing it through a cylindrical lens. However, expanding a typically Gaussian laser beam into a line results in a Gaussian intensity distribution along the length of the projected line. This in turn results in thresholding problems.
U.S. Pat. No. 4,826,299 to Powell describes a single optical element which projects a laser line having uniform intensity along its length. Generally, only one surface of the optical element is responsible for this effect. The divergence of the projected beam is primarily a function of the shape of that surface, more particularly its radius of curvature and conic constant. A secondary influencing factor is the refractive index of the material of the element.
U.S. patent application Ser. No. 08/242,863 now U.S. Pat. No. 5,523,889 describes an optical lens which consists of a number of the elements of the Powell US Patent, supra, mounted into a star-like arrangement, for expanding a laser beam into a corresponding star-shaped projected image.
For inspection purposes, especially in regard to objects having circular symmetry, it is sometimes desirable to project a beam of light in the shape of a "D" so that characterization of the object in question can be realised. The object would generally have a reference surface and the characterization would involve such matters as the degree of symmetry associated with the object, its actual size, alignment etc.
It is an object of the invention to provide a lens enabling the expansion of a laser beam into a semi-circular or similar shape having an outline resembling the letter D.